This invention relates to novel graft copolymers prepared by reacting isobutylene-conjugated diene copolymers with anionically polymerizable monomers, such as styrene. More particularly, the invention relates to an improvement in the use of polymer lithiation to prepare the novel graft copolymers.
The use of alkyl lithium polymer complexes in graft polymerization reactions is known and disclosed, for example, in U.S. Pat. Nos. 3,451,988 and 3,458,568 and in pending application Ser. No. 562,826 filed Mar. 28, 1975 by A. W. Langer. A problem encountered in such processes has been the achievement of high grafting efficiency when elastomeric isobutylene copolymers are used as the grafting site, that is, providing a product where a substantial portion of the reacted monomer is converted to graft copolymer as opposed to homopolymerized monomer.
In accordance with the present invention, there has been discovered an improved process for grafting styrene monomer in high grafting efficiencies onto an isobutylene-conjugated diene copolymer elastomer which comprises the steps of: (a) forming a solution, or cement, of the elastomer polymer in an inert hydrocarbon solvent; (b) treating said solution with an organo lithium-polyamine composition whereby a lithiated polymer complex is formed; (c) treating said complex with from about 2 to 50 volume % of an organic ether compound at a temperature of at least about 20.degree. C. until any unreacted organo lithium-polyamine is decomposed; and (d) contacting styrene monomer with the product of step (c) whereby a novel graft copolymer of said elastomer and said monomer is formed wherein at least a major portion of the reacted styrene monomer is present in the copolymer.
The elastomers useful in the present invention may generally be described as those rubber hydrocarbon elastomeric isobutylene-conjugated diene copolymers having a conjugated diene content between about 5 and 50, preferably 5 to 35, mole percent. Illustrative of such elastomers are copolymers of isobutylene and conjugated multiolefins having about 4 to 14, preferably 5 to 9, carbon atoms such as iosprene, piperylene, butadiene, 2,3-dimethylbutadiene, cyclopentadiene, methylcyclopentadiene, cyclohexadiene and the like. These copolymers generally will have a viscosity average molecular weight of about 50,000 to 800,000.
Solvents for the elastomers generally comprise the saturated aliphatic or cycloaliphatic inert hydrocarbon solvents such as pentane, hexane, heptane, isooctane, cyclopentane, cyclohexane, decane hexadecane and the like, with cyclohexane being a particularly preferred solvent.
The use of organo lithium and tertiary chelating polyamine catalysts is, per se, known in the art and described in detail, for example, in pending application Ser. No. 562,826 of A. W. Langer, Jr., filed Mar. 28, 1975. Generally, the organo lithium compound contains 1 to 15 carbon atoms, preferably 1 to 8 carbon atoms and the tertiary chelating polyamine is monomeric or polymeric in nature, the monomeric units containing about 3 to 50 carbon atoms.
In general, alkyl lithiums are preferred but cycloalkyl, aryl, or arylalkyl lithium compounds are also suitable. Examples of organo lithium compounds for use in this invention are methyllithium, n-butyllithium, cyclooctyllithium, dodecylithium, 2-methyl-butyllithium, phenyllithium, benzyllithium, sec-butyllithium, allyllithium, vinyllithium, and the like with n-butyllithium and sec-butyllithium being preferred. In conjunction with the organolithium compound is employed an amine containing at least 2 nitrogen atoms which is capable of chelating with the organolithium component of the catalyst. Monomeric diamines and triamines are preferred, however, the polyamine may contain 4 or more nitrogen atoms and may be polymeric rather than monomeric in nature. Examples of preferred polyamines include N,N-dimethyl-N', N'-diethyl-1,2-ethanediamine; N,N,N',N'-tetramethyl-1,2-ethanediamine; N,N,N',N'-tetraethyl-1,2-ethanediamine; N,N,N',N'-tetramethyl-1,3-propanediamine; N,N,N',N'-tetramethyl-1,2-propanediamine; N,N,N',N'-tetramethyl-1,4-butanediamine; N,N,N',N'-tetramethyl-1,2-cyclohexanediamine, 1,2-dipyrolidylethane, sparteine and the like. Particularly preferred for use in the present invention are N,N,N',N'-tetramethyl-1,2-ethanediamine hereinafter referred to as TMEDA and trans-N,N,N',N'-tetramethyl-1,2-cyclohexanediamine.
The ethers employed in the process of the present invention include organic aliphatic, aromatic or heterocyclic monoethers and polyfunctional ethers, such as diethers, triethers and tetraethers. Generally, the ethers are compounds which are normally liquid at the temperature of the ether treatment step. Preferably, the ether employed will have a boiling point of from about 30.degree. to 100.degree. C. to facilitate recycling, processing, solvent removal and similar procedures. Examples of such ethers are dimethyl ether, diethyl ether and higher dialkyl ethers, the alkyl groups having up to about 8 carbon atoms as further exemplified by di-n-butyl ether, diisobutyl ether, di-n-heptyl or di-octyl ether and the like. These ethers may also be mixed saturated ethers such as methyl-t-butyl ether, ethyl-n-amyl ether and the like. Acetals and ketals such as dimethoxymethane, 2-methoxytetrahydrofuran, 1,3-dioxolane and 2,2-dimethoxypropane are also suitable. Heterocyclic ethers such as tetrahydrofuran, which is particularly preferred in the present invention, and 2-methyltetrahydrofuran and 3-methyltetrahydrofuran, and di-, tri- and tetraethers such as dioxane, diethylene glycol diethyl ether, diethylene glycol dimethyl ether, diethylene glycol dibutyl ether are also suitable. Aromatic ethers are exemplified by benzyl ether, benzylmethyl ether, phenylmethyl ether and the like.
In the present invention, outstanding results have been achieved utilizing styrene. It is believed that other anionically polymerizable monomers may find utility in the present invention and these monomers are exemplified by C.sub.4 -C.sub.12 conjugated diolefins such as butadiene, 1,3-isoprene, 2-alkyl-1,3-butadiene, 2-phenylbutadiene-1,3 and the like, aromatic vinylic hydrocarbons, such as alphamethylstyrene, p-methylstyrene, p-chlorostyrene, p-(t-butyl)styrene, vinyltoluene as well as mixtures of such monomers. Also of possible utility are alkylene oxides such as propylene oxide, butadiene monoxide as well as alkyl acrylates and methacrylates such as methylmethacrylate, ethylacrylate, butylacrylate and the like. Other potentially suitable monomers are acrylonitrile, dialkyl acrylamides, vinyl heterocyclics, and similar anionically polymerizable monomers.
In the process of the present invention the ether treatment step has been found to be highly effective in promoting the grafting efficiency when the isobutylene copolymer elastomers as described herein are employed as the base polymer. Grafting efficiencies on the order of at least 50% and usually in the range of 70 to 80% are readily achieved in accordance with the present invention. The term "grafting efficiency" as used herein refers to the portion of reacted styrene monomer which is converted to copolymer, that is, the weight percent monomer converted to graft copolymer relative to total monomer conversion. It has been found that in the absence of the ether treatment step a substantial quantity of monomer will be converted to homopolymer without the desired graft copolymer being formed. It is believed that the ether functions by decomposing any unreacted organolithium polyamine complex present prior to addition of the monomer. If the catalyst complex is left unreacted it will adversely affect the grafting reaction by promoting conversion of the monomer to homopolymer.
The ether treatment is carried out so that there is employed about 2 to 50 volume % based on the total volume of reaction mixture, presently about 5 to 30 volume % of ether. Moreover, it has been found that the temperature during the ether treatment step should be at least about 20.degree. C., otherwise poor grafting efficiencies are likely to result. A temperature range of about 20.degree. to 50.degree. C., particularly 40.degree.-50.degree. C., is generally suitable for the ether treatment step, particularly when the preferred ether, tetrahydrofuran, is employed. At the preferred temperature range, the ether treatment step is carried out over a period of about 30 minutes to 4 hours.
The grafting reaction is carried out using conventional techniques once the ether treatment step has been completed. The grafting reaction may be carried out conveniently at room temperature and pressure. Generally, temperatures in the range of -100.degree. to 150.degree. C., preferably 0.degree. to 100.degree. C., are used with pressure ranging from atmospheric pressure to 5000 psig or higher as may be required when the monomer is gaseous.
The styrene monomer is allowed to remain in contact with the polymer complex for about 0.1 to 10 hours, preferably 0.5 to 4 hours. The quantity of styrene added is quite variable depending on the desired properties of the graft product desired. Generally, the range of styrene added is from about 10 to 400 parts per hundred parts of isobutylene copolymer backbone. The reaction is terminated by adding small amounts of water or an alcohol, such as methanol, isopropanol, n-butanol and the like. The polymer is then recovered by adding excess alcohols, such as those mentioned, or ketones such as acetone or methyl ethyl ketone. The precipitated graft copolymer product is then filtered and washed with more alcohol or other suitable solvents, and dried.
The styrene grafted products of the present invention are considered to be novel thermoelastomer products and, as such, they constitute a further embodiment of the present invention. As thermoelastomers these novel products have the elastic and resilient properties of rubbers thereby being adaptable to thermoplastic processing techniques such as injection molding, extrusion blow molding or vacuum forming for these novel copolymers. The styrene content of the graft copolymer product may vary from about 10 to 80 weight percent. The preferred ranges are a function of the properties desired. For a thermoelastic product having predominantly rubbery properties such as high elongation and low compression set or tensile set, the preferred range is 20 to 40 weight percent. For thermoelastics with predominantly plastic-like properties, that is, relatively higher tensile strength and flexural modulus but less resiliency and high compression set, the preferred range of styrene content in the finished product is 40 to 70 weight percent styrene.
Preferred products include styrene modified isobutylene-diene copolymers wherein the diene content of the isobutylene-diene polymer backbone is 5 to 35 mole percent. Preferred dienes are isoprene, butadiene, cyclopentadiene, methyl-cyclopentadiene, 2-3-dimethylbutadiene and piperylene.